Archaeocyathan buildups within an entirely siliciclastic succession: New discovery in the Toyonian Lalun Formation of northern Iran, the Proto-Paleotethys passive margin of northern Gondwana

In striking contrast to other Lower Cambrian reefs, the archaeocyathan buildups of the Alborz flourished in a completely siliciclastic setting and lack the skeletal calcimicrobes which dominated the Lower Cambrian reefs. Abrupt lateral and vertical facies changes to colored shale and fluvial red beds, the presence of infiltrated shale between the skeletons, and in central cavities and the intervallum of the archaeocyaths suggest highly turbulent and turbid incoming water with abnormally high concentrations of fine siliciclastic material during reef development. High concentrations of fine, suspended siliciclastics could well have prevented the light penetration which was necessary for calcimicrobial growth. In addition, the relatively small size of the archaeocyaths and absence of reef dwellers is very likely a consequence of the high terrigenous mud content and, perhaps, below-normal salinity of the ambient sea water. In the absence of calcimicrobes and suspension feeder metazoans in the dark and muddy water of the mid-estuarine setting, archaeocyathans became the only bioconstructors of the Iranian buildups. Abnormally high concentration of nutritious fine siliciclastics suggests that photosymbionts and oligotrophic conditions were not needed by archaeocyaths; particular hydrodynamic conditions along with high nutrient flux, rather than light, were essential for archaeocyathan communities.